Create a new header src/mongo/db/exec/expression/evaluate.h and define a namespace mongo::exec::expression with a set of functions

Value evaluate(const ExpressionMap& expr, const Document& root, Variables* variables)

one for each

• ExpressionMap
• ExpressionReduce
• ExpressionFilter

The body of the functions will be in a src/mongo/db/exec/expression/evaluate_map_reduce.cpp and will be taken from the evaluate() method, and the evaluate() method will be changed so that it calls the new method.